In respect of conventional acoustic musical instruments, as illustrated in FIG. 1, there is a sound source 10 under control of a musician 20, wherein an output S1 from the sound source 10 is conveyed via a coupling arrangement 30 to generate an acoustic output S2 which is eventually appreciated as an acoustic sound by the musician 20 and potentially other persons listening to the acoustic output S2, for example an audience. The coupling arrangement 30 can be passive or active. “Active” corresponds to the output S1 being subject to amplification to generate the acoustic output S2.
An example of a passive implementation of the coupling arrangement 30 is a sound board of an acoustic piano; the sound source 10 in such case corresponds to a keyboard, a hammer mechanism, and a metal frame with piano “strings” stretched thereacross, wherein the keyboard receives from the musician 20 an input force via the keyboard to actuate the hammer mechanism to excite the “strings” into resonance to generate the output S1. The coupling arrangement 30 implemented in a passive mode is beneficially analyzed, namely represented, as a series of resonances R1 to Rn. The resonances R1 to Rn have corresponding Q-factors Q1 to Qn, corresponding coupling coefficients k1 to kn, and corresponding center frequencies f1 to fn. The resonances R1 to Rn are included within a frequency range of interest, for example 20 Hz to 20 kHz. Thus, the emitted sound S2 is susceptible to being mathematically derived from the output S1 by way of Equation 1 (Eq. 1):
                              S          2                =                              ∑                          i              =              1                        n                    ⁢                                    k              i                        ⁢                          R              i                        ⁢                          S              1                                                          Eq        .                                  ⁢        1            In practice, suitable selection of the resonances R and their associated resonant frequencies, together with the coupling coefficients k are vitally important when manufacturing a quality acoustic musical instrument, for example when constructing a quality grand piano or a quality acoustic guitar, because the coupling arrangement 30 causes distinct coloration of the output S1 which enables the acoustic instrument to be recognized and appreciated by the musician 20 and potentially other persons listening to the acoustic output S2. For accurately describing an acoustic instrument, the number n of resonances R employed in Equation 1 (Eq. 1) can be potentially very large, for example several hundred to several thousands.
In a high-fidelity sound reproduction system, the sound source 10 is, for example, a CD player including a high-quality DAC output or similar to generate the output S1, and the coupling arrangement 30 is implemented as an amplifier arrangement coupled to a loudspeaker arrangement and is carefully designed to be as accurate as possible so that the acoustic output S2 is as faithful a reproduction of the output S1 as technically possible. Special measures, for example use of electrostatic speakers and class-A solid-state linear amplifiers, are sometimes employed to achieve most accurate sound reproduction in top quality high fidelity sound reproduction systems.
Another example of an active implementation of the coupling arrangement 30 is a thermionic electron tube amplifier 50 with an associated loudspeaker arrangement 60 as illustrated in FIG. 2. The thermionic electron tube amplifier 50, also known as a “valve” amplifier, is arranged in operation to receive an electrical signal as the output S1 from a pickup of an electric guitar 10, and to amplify the output S1 to generate a corresponding acoustic output S2 from the loudspeaker arrangement 60. Well known commercial companies such as Marshall Amplification (United Kingdom), Peavey (United States of America), Fender (United States of America) manufacture such active sound amplification apparatus, although there are many alternative manufacturers of active sound amplification apparatus in the World competing for market share; “Marshall”, “Peavey” and “Fender” are registered trade marks (®). Musicians skilled in playing electric guitars contemporarily greatly enjoy employing valve amplifiers and associated loudspeaker arrangements for generating the acoustic output S2. Such enjoyment derives from considerable sound coloration introduced in operation by such valve amplifiers and associated loudspeaker arrangements. When sound coloration occurs, the coefficients k and Q-factors Q of the resonances R in Equation 1 (Eq. 1) are contemporarily perceived to vary considerably over the frequency range of interest.
Certain constructions of the loudspeaker arrangement 60, namely including one or more loudspeaker driver units 100, referred to as “drivers”, and their associated one or more cabinets 110, have certain distinctive sound coloration characteristics. In contradistinction, in the case of high-fidelity apparatus, it is desirable that the sound coloration should be small as possible. The distinctive sound coloration characteristics are potentially influenced by one or more of following factors:                (a) a physical shape and size of the one or more cabinets 110;        (b) a material from which the one or more cabinets 110 are fabricated, whether or not a volume enclosed by walls of the one or more cabinets 110 are at least partially filled with sound absorbing materials, whether or not the walls of the one or more cabinets 110 present the volume with irregular surface topology or one or more planar surfaces, and whether or not the one or more cabinets 110 are of a back-vented configuration or infinite-baffle closed construction;        (c) a material from which diaphragms of the one or more loudspeaker driver units 100 are manufactured, a geometrical shape of the diaphragms, and an elasticity of their spider mounts and roll surrounds which are employed to center and support the diaphragms; and        (d) a manner in which the one or more loudspeaker driver units 100 are spatially disposed in the one or more cabinets 110.        
For example, it is conventional practice to construct the one or more cabinets 110 from solid wood, plywood, medium density fiber board (MDF) or chipboard panels which are at least partially filled with acoustic wadding, and the one or more driver units 100 are manufactured with diaphragms manufactured from stiffened impregnated paper or cloth. Occasionally, more exotic materials such as Titanium, Kevlar or Carbon fiber are employed for fabricating the diaphragms; “Kevlar” is a registered trademark (®).
It has become contemporary practice to provide musicians with a musician-selectable simulation, namely synthesis or emulation, of different amplification system colorations in their sound amplification equipment; for example, such selection is contemporarily provided as a user-selectable option by way of a rotatable switch or equivalent on amplifier units. These simulations, namely amplifier emulations, are optionally provided for example in a context of “combo units” wherein the valve amplifiers 50 and the one or more loudspeaker driver units 100 are housed together as integrated apparatus. These emulations may alternatively be implemented via processing software when processing recorded signals for producing musical products such as compact discs (CD) and sound files for subsequent distribution to customers. The musicians are thereby able to select between supposedly different types of loudspeaker arrangements and associated thermionic electron tube amplifiers to achieve a desired musical effect, namely sound coloration, for example in response to an epoch of music being performed. It is contemporary practice that the simulations be conventionally derived from a measurement and associated analysis of an input signal, equivalent to the output S1, to a thermionic electron tube (“valve”) amplifier and a corresponding acoustic output, equivalent to the acoustic output S2, from a speaker arrangement coupled to the amplifier, wherein the acoustic output S2 is measured using a high quality microphone which is conventionally assumed to be substantially devoid of coloration effects; by analyzing the acoustic output derived from the microphone relative to the input signal S1 to the valve amplifier by way of an impulse pulse response and/or a swept frequency response and performing a form of mathematical processing, for example a convolution or de-convolution, pursuant to Equation 1 (Eq. 1), it is feasible to provide aforesaid simulations, namely emulations. However, such an approach often does not provide a sufficiently accurate simulation, namely synthesis or emulation, in view of highly complex sound coloration process which occur in practice for a whole variety of reasons.
Contemporary combo amplifiers typically include an amplifier unit and one or more loudspeakers within a housing. Typically, the amplifier unit is usually implemented using thermionic electron tubes and/or analogue solid-state devices for providing signal amplification. Moreover, contemporary amplifier emulators attempt to simulate a sound of valve amplifiers or solid-state amplifiers using digital signal processing (DSP) and/or solid-state circuits. The emulators are often implemented in a contemporary context using software executable upon computing hardware. However, musicians find that contemporary simulation, namely emulations, are not sufficiently realistic and representative, despite contemporarily great care being taken when measuring characteristics of sound amplification systems
In a published international PCT application no. WO 00/28521 (PCT/GB99/03753, “Audio dynamic control effects synthesizer with or without analyzer”, Sintefex Audio LDA), there is described a method and apparatus for applying a gain characteristic to an audio signal. Data storing a plurality of gain characteristics at a plurality of different levels is stored in data storage means. The amplitude of an input signal is repeatedly assessed and from this a gain characteristic to be applied to the input is determined.